"Novel" or "atypical" antipsychotic medications constitute the current standard of care for the treatment of many serious mental illnesses because they target both positive and negative symptoms and have fewer extrapyramidal side effects than earlier medications. However, there is growing concern that these psychiatric benefits are offset by serious negative medical consequences related to weight gain including obesity-related cardiovascular risk, insulin resistance, diabetes, and diabetic ketoacidosis. The risk/benefit ratio of atypical antipsychotic regimens could be improved if behavioral interventions helped persons with serious mental illness to reduce weight, sustain weight loss, and achieve better fitness. HIV prevention research has shown that cognitive-behavioral small-group interventions-especially when combined with mechanisms that provide peer and structural/environmental risk reduction support-can help persons change even longstanding risk patterns. Following a period of formative ethnographic research related to intervention development, this project will evaluate the effects of a behavior change intervention modeled after approaches shown effective in the HIV prevention field. In a randomized controlled trial design, 20 community group homes providing residence to persons with serious mental illness will be assigned to either an experiemental intervention that seeks to decrease residents'weight and increase exercise, or to a time- matched attention control program. The intervention, based on social-cognitive theoretical principles, will include a 20-session small-group component conducted in each group home focused on diet and exercise. Social leaders identified in each home will be trained to support others'efforts, and environmental change supports will be introduced in the group home settings. At baseline and a 3 post-intervention followup points over 12 months, data will be collected to measure change in: (1) participants'diet and exercise patterns using self-report and observational methods including daily pedometer and food monitoring for one-week periods;(2) clinical health indicators (weight, body mass index, body fat distribution, pulse rate, and blood pressure;(3) laboratory tests measuring blood glucose, cholesterol HDL, LDL, triglicerides, and other indicators;and (4) ancillary domains including psychological well-being, health quality of life, and self- reported antipsychotic medication adherence. We hypothesize that residents in experimental condition group homes will show greater average weight loss, greater reduction in body mass index, and positive effects across other behavioral, clinical, physiological, and ancillary measures than residents in control condition group homes at each followup point. If successful, this research will identify a pratical behavioral intervention approach that may be able to offset significant medical risk factors often experienced by persons with serious mental illness, especially those on novel antipsychotic medications.